Lithium-manganese ferromagnetic ferrite core



United States Patent This invention relates to a lithium-manganeseferromagnetic ferrite core which exhibits a B /B ratio of at least about0.90 and a coercive force between about 0.3

and 0.9 oersted. The invention relates also to a method of manufacturethereof. The ferrite core of the invention is particularly adapted to beused as a memory element in coincident current memories of digitalelectronic computers, and as a saturable reactor in other electronicapparatus. The terms core and body are used interchangeably in thisdocument to refer to a sintered mass of ferrite particles.

There have been proposed ferrite cores which exhibits a B /B ratiogreater than 0.7 and which consist essentially of a lithium-manganeseferrite. Such lithium-manganese ferrite cores have been prepared bysintering a mixture consisting essentially of lithium oxide, manganeseoxide and ferric oxide. The lithium-manganese ferrites generally exhibita coercive force H greater than 1.0 oersted, and usually greater than5.0 oersteds.

Further improvements to these lithium-manganese ferrite cores aredesirable, particularly in providing cores which are particularlyadapted to use as memory elements in coincident current memories. Suchapplications require a combination of desired properties; particularly B/B ratio of about 0.90 and greater, a coercive force between about 0.3and 0.8 oersted, and a relatively high Curie temperature. In addition,the cores should be capable of being fabricated by sintering attemperatures of 1250 C. or less in order that the cores be manufacturedeconomically.

An object of this invention is to provide an improved ferromagneticferrite body.

Another object is to provide an improved method for fabricating aferromagnetic ferrite body.

A further object is to provide a lithium-manganese ferrite core whichexhibits a B /B ratio of at least about 0.90 and a coercive forcebetween about 0.3 and 0.8 oersted.

In general, the objects of this invention are achieved with a ferritecore which comprises a ceramic prepared by sintering a mixtureconsisting essentially of:

M01 percent Lithium fluoride 1 to 5 Manganese oxide to 59 Ferric oxide40 to 49 at temperatures between about 1150 and 1250 C. If the sinteringis carried out in an air atmosphere, either the core should be airquenched, or the core should be annealed in an atmosphere of a neutralgas, such as nitrogen, containing 0.5 to 2.0 volume percent oxygen. Thesintering may be also carried out in an atmosphere of nitrogencontaining 0.5 to 2.0 volume percent oxygen and then slowly cooled inthis atmosphere- By substitutr ing lithium fluoride for the lithiumoxide used in the prior art method, and sintering the mixture asdescribed, cores are produced which achieve the objects of theinvention.

3,146,205 Patented Aug. 25, 1964 Example.-A ferromagnetic core of theinvention may be prepared by the following procedure. Mix a raw batch ofthe following ingredients.

Ingredient: Molar percent LiF Mallinkrodt CP powder 3.5 MnO, as MnCOBakers analyzed reagent grade 51.5 Fe O as Fishers analyzed reagentgrade, F203 The raw batch is attrited in methanol in a steel ball millfor about four hours and then dried at about 100 C. and screened. Theattrited mixture is calcined for about one half hour in air at about 800C. The calcined batch is attrited in methanol for about twenty hours andthen dried. About 3.5 weight percent of a suitable organic binder isevenly dispersed through the dry calcined batch. One suitable binder isFlexalyn in methyl ethyl ketone, which binder is marketed by theHercules Powder Company, Wilmington, Delaware. The calcined batch withthe binder added is screened through an mesh screen. The screenedcalcined batch is then pressed into toroidal cores, about 0.20" 0D. x0.15" 1D. x 0.05 thick. The pressed cores are then sintered for about 2hours at about 1150 C. in air and then air quenched by removing thecores from the furnace directly into room temperature.

A core prepared according to the example exhibits a B /B ratio of about1.00, a coercive force H of about 0.35, and a Curie temperature of about340 C. The data for the core of the example appears in the table alongwith the data for other cores prepared according to the invention.

The following compositional and procedural variations may be made fromthat set forth in the example. The composition of the raw batch may bevaried as described above. In addition, up to about 10 mol percent ofZnO and/ or CdO may be substituted for MnO in the raw batch.

' Such substitutions lower the coercive force H and lower the Curietemperature of the final product.

The composition of the final product is uncertain. Chemical analysis ofthe final product indicates that almost all of the fluorine content ofthe batch is lost during the sintering step. Also, a portion of themanganese in the raw batch is lost during the sintering step. Theselosses seem to indicate that there is a chemical reaction in whichmanganese fluoride is formed and volatilized during the sintering step.This is reasonable since manganese fluoride has a relatively lowvolatilization temperature compared with the other possible compoundswhich may be present during sintering.

Except for lithium fluoride, the batch may be com pounded of theconstituent metal oxides or of compounds which yield the constituentmetal oxides by chemical reaction during the calcining of the raw batchor during the sintering of the core. Typical compounds may be, forexample, carbonates, oxides, or acetates of manganese and iron. A highdegree of purity is desirable, preferably the chemically pure grade ofchemicals.

In the example, the steps of mixing, calcining, attriting, drying, andscreening are designed to provide an intimate mixture of theingredients, and to remove gases, water, and volatile organic mattercontained in the raw batch. These steps are not critical. Any procedurewhich provides a dry, intimate mixture of the ingredients issatisfactory.

In the example, the calcining step is important. The calciningtemperature may be between 700 and 900 C., The

were obtained from 60 c.p.s. loop traces on tol'oids which were woundwith a primary of 6 turns and a secondary of 4 turns of copper wire. A60 c.p.s. sine wave drive current was applied to the primary. Thesecondary was connected to the input of an amplifier. The output of thebut is preferably near the middle of the range. 5 calcining time is notcritical, although shorter times are amplifier was connected to anintegrating circuit. The preferred with higher calcining temperatures.Air is the output of the integrating circuit was applied to the verticalpreferred calcining atmosphere. Other atmospheres havplates of anoscilloscope. The horizontal trace of the mg oxidizing characteristicssimllar to that of air at the oscilloscope was proportional to the drivecurrent applied calcining temperature may also be used. 10 to theprimary on the sample. The drive current was be- In the example,attfitlng the a in d batch, addition of tween about 0.2 and 2.0 amperes.Curie temperature a binder, screening, and pressing are not critical tothe measurements indicate that all of the cores of the invenmagneticproperties of the final product; however, aproper tion have a Curietemperature greater than 300 C.

Table M01 percent 1,150 0., 2 hrs., 1,200 C., 1hr., 1,250 0., 1 hr.,1,150 C., 2111's.,

air quenched air quenched air quenched 0.4% Oz 99.6%

LiF M110 F 03 Br/B, H Jar/Ba H 1343. 1L, B4B. H,

selection should be made in order to obtain the desired shape and sizeof product with a minimum distortion. Besides toroidal cores, othershapes such as magnetic memory plates and transfiuxor cores may beprepared. See a description of ferrite core fabrication processes in G.S. Hipskind et al., Processing and Testing Rectangular Loop Cores, RCAEngineer, volume 11, No. 6, 1957, pages 9 to 13.

In the example, the sintering temperature may be between about 1150" and1250 C. The sintering time is not critical. Any sintering timesufficient for complete sintering of the core is adequate. One to 24hours, preferably 2 hours, have been found to be convenient firingtimes.

The sintering atmosphere and the cooling or annealing atmosphere areimportant. The cores described herein may be prepared by sintering inair and then air quenching the cores as described in the example. By oneprocedure for carrying out this alternative the cores, which are on ahot setter plate, are removed from the furnace at about the sinteringtemperature directly into a room temperature ambient. The cores aredumped from the hot setter plate to a setter plate at room temperatureand permitted to cool. Generally, the cores are on a setter plate whenthey are sintered, and any method of rapid cooling in air to roomtemperature may be used for quenching.

Alternatively, the .cores may be sintered in an air atmosphere asdescribed above and then annealed in an atmosphere consistingessentially of a neutral gas containing 0.5 to 2.0 volume percent oxygengas. Some suitable neutral gases are nitrogen, argon, neon, helium, ormixtures thereof. The annealing temperature may be about 900 to 1100 C.The annealing time is not critical. One to hours, preferably 2 hours, isa convenient time period for annealing. This alternative may be carriedout in one or two steps.

Alternatively, the cores may be sintered in an atmosphere consistingessentially of a neutral gas containing 0.5 to 2.0 volume percent oxygengas and then slowly cooled to room temperature (annealed) in the sameatmosphere.

The table tabulates some of the properties of some typical cores of theinvention. The table indicates the compositon and method of preparationand gives the coercive force H in oersteds and B /B ratio to the seconddecimal place. The values of H and l?,/B ratio What is claimed is:

1. A ferromagnetic ferrite having a B /B ratio of at least 0.90 formedby calcining a mixture consisting essentially of:

M01 percent Lithium fluoride 1 to 5 Manganese oxide 50 to 59 Ferricoxide 40 to 49 at about 700 to 900 C. in an oxidizing atmosphere,shaping a core from said calcined mixture, and sintering said core for 1to 24 hours at about 1150 to 1250 C.

2. A ferromagnetic ferrite having a B /B ratio of at least about 0.90and a coercive force of about 0.3 to 0.8 oersted formed by calcining amixture consisting essentially of:

M01 percent Lithium fluoride l to 5 Manganese oxide 50 to 59 Ferricoxide 40 to 49 Lithium fluoride 1 to 5 Manganese oxide 50 to 59 Ferricoxide 40 to 49 at about 700 to 900 C. in an oxidizing atmosphere,shaping a core from said calcined mixture, sintering said core for 1 to24 hours in air at about 1150 to 1250 C., and then annealing saidsintered core for 1 to 10 hours at about 900 to 1100 C. in an atmosphereconsisting essentially of a neutral gas containing about 0.5 to 2.0volume percent oxygen.

4. A ferromagnetic ferrite having a B /B ratio of at least about 0.90and a coercive force of about 0.3 to 0.8 oersted formed by calcining amixture consisting essentially of:

M01 percent Lithium fluoride 1 to 5 Manganese oxide S0 to 59 Ferricoxide 40 to 49 0.8 oersted formed by calcining a mixture consistingessentially of:

M01 percent Lithium fluoride 2.5 to 3.5 Manganous oxide 51.5 to 57.5Ferric oxide 40 to 45 at about 700 to 900 C. in an oxidizing atmosphere,shaping a core from said calcined mixture, and sintering said core for lto 24 hours at about 1150 to 1250 C.

6. A ferromagnetic ferrite having a B,,/B ratio of at least about 0.90and a coercive force of about 0.3 to 0.8 oersted formed by calcining amixture consisting essentially of:

M01 percent Lithium fluoride 2.5 Manganous oxide 52.5 Ferric oxide 45.0

at about 700 to 900 C. in an oxidizing atmosphere, shaping a core fromsaid calcined mixture, sintering said core for 1 to 24 hours at about1150 to 1250 C. in air, and then air quenching said sintered core.

7. A ferromagnetic ferrite having a B /B ratio of at least about 0.90and a coercive force of about 0.3 to 0.8 oersted formed by calcining amixture consisting essentially of:

M01 percent Lithium fluorid 3.5 Manganous oxide 51.5 Ferric oxide 45.0

at about 700 to 900 C. in an oxidizing atmosphere, shaping a core fromsaid calcined mixture, sintering said core for l to 24 hours in air atabout 1150 to 1250 C., and then air quenching said sintered core.

8. A ferromagnetic ferrite having a B /B ratio of at least about 0.90and a coercive force of about 0.3 to 0.8 oersted formed by calcining amixture consisting essentially of:

M01 percent Lithium fluoride 2.5 Manganous oxide 52.5 Ferric oxid 45.0

at about 7 00 to 900 C. in an oxidizing atmosphere, shaping a core fromsaid calcined mixture, sintering said core for 1 to 24 hours in air atabout 1150 to 1250 C., and then annealing said sintered core for l to 10hours at about 900 to 1100 C. in an atmosphere consisting essentially ofnitrogen containing about 0.5 to 2.0 volume percent oxygen.

9. A ferromagnetic ferrite having a B /B ratio of at least about 0.90and a coercive force of about 0.3 to 0.8 oersted formed by calcining amixture consisting essentially of:

M01 percent Lithium fluoride 3.5 Manganous oxide 51.5 Ferric oxide 45.0

at about 7 00 to 900 C. in an oxidizing atmosphere, shaping a core fromsaid calcined mixture, sintering said core for 1 to 24 hours in air atabout 1150 to 1250 C., and then annealing said sintered core for l to 10hours at about 900 to 1100 C. in an atmosphere consisting essentially ofnitrogen containing about 0.5 to 2.0 volume percent oxygen.

6 10. A method for preparing a ferromagnetic ferrite having a B /B ratioof at least about 0.90 comprising calcining a mixture consistingessentially of:

M01 percent Lithium fluoride 1 to 5 Manganese oxide 50 to 59 Ferricoxide- 40to 49 at about 700 to 900 C. in air, shaping a core from saidcalcined mixture, and then sintering said core at about 1150 to 1250 C.

11. A method for preparing a fenromagnetic ferrite having. a B /B ratioof at least about 0.90 and a coercive force of about 0.3 to 0.8 oerstedcomprising calcining a mixture consisting essentially of:

M01 percent Lithium fluoride 1 to 5 Manganese oxide 50 to 59 Ferricoxide 40 to 49 at about 700 to 900 C. in air, shaping a core from saidcalcined mixture, sintering said core for l to 24 hours in air at about1150 to 1250 C., and then air quenching said sintered core.

12. A method for preparing a ferromagnetic ferrite having a B,./B ratioof at least about 0.90 and a coercive force of about 0.3 to 0.8 oerstedcomprising calcining a mixture consisting essentially of:

M01 percent Lithium fluoride 1 to 5 Manganese oxide 50 to 59 Ferric oxid40 to 49 at about 700 to 900 C. in air, shaping a core from saidcalcined mixture, sintering said core for 1 to 24 hours in air at about1150 to 1250 C., and then annealing said core for 1 to 10 hours at about900 to 1100 C. in an atmosphere consisting essentially of a neutral gascontaining 0.5 to 2.0 volume percent oxygen.

13. A method for preparing a ferromagnetic ferrite having a B /B ratioof at least about 0.90 and a coercive force of about 0.3 to 0.8 oerstedcomprising calcining a mixture consisting essentially of:

M01 percent Lithium fluoride 1 to 5 Manganese oxide 50 to 59 Ferric ox40 to 49 at about 700 to 900 C. in air, shaping a core from saidcalcined mixture, sintering said core for 1 to 24 hours at about 1150 to1250 C. in an atmosphere consisting essentially of a neutral gascontaining 0.5 to 2.0 volume percent oxygen, and then annealing saidcore for 1 to 10 hours at about 900 to 1100 C. in said atmosphere.

14. A method for preparing a ferromagnetic ferrite having a l5 /B ratioof at least about 0.90 and a coercive force of about 0.3 to 0.8 oerstedcomprising calcining a mixture consisting essentially of:

M01 percent Lithium fluoride 2.5 to 3.5 Manganous oxide 51.5 to 57.5Ferric oxide 40 to 45 at about 700 to 900 C. in air, shaping a core fromsaid calcined mixture, sintering said core for 1 to 24 hours in air atabout 1150 to 1250 C., and then air quenching said sintered core.

15. A method for preparing a ferromagnetic ferrite having a B /B ratioof at least about 0.90 and a coercive force of about 0.3 to 0.8 oerstedcomprising calcining a mixture consisting essentially of:

M01 percent Lithium fluoride 2.5 to 3.5 Manganous oxide 51.5 to 57.5Ferric oxide 40 to 45 at about 700 to 900 C. in air, shaping a core fromsaid calcined mixture, sintering said core for 1 to 24 hours 7 in air atabout 1150 to 1250 C., and then annealing said core for 1 to 10 hours atabout 900 to 1100 C. in an atmosphere consisting essentially of nitrogencontaining 0.5 to 210 volume percent oxygen.

16. A method for preparing a ferromagnetic ferrite having a B /B ratioof at least about 0.90 and a coercive force of about 0.3 to 0.8 oerstedcomprising calcining a mixture consisting essentially of:

M01 percent Lithium fluoride Manganous oxide 51.5 to 57.5 Ferric oxide40 to 45 at about 700 to 900 C. in airTshaping a core from said calcinedmixture, sintering said core for 1 to 24 hours at about 1150 to 1250" C.in an atmosphere consisting essentially of nitrogen containing 0.5 to2.0 volume percent oxygen, and then annealing said core for 1 to 10hours at about 900 to 1100 C. in said atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS

1. A FERROMAGNETIC FERRITE HAVING A BR/BS RATIO OF AT LEAST 0.90 FORMEDBY CALCINING A MIXTURE CONSISTING ESSENTIALLY OF: